In electronic apparatuses such as digital cameras, cell telephones and printers, a circuit board obtained by joining flexible circuit boards (such as flexible printed circuit boards (FPC)) with one another is used in many cases. These electronic apparatuses are becoming small in size and the need for mutually connecting circuit boards having wirings at a fine pitch is increasing.
Connection of a circuit board to another circuit board has been conventionally performed by providing solder bumps on the connection parts of the circuit board, and contacting and soldering the connection parts to the electrodes of another circuit board. However, pitch between connection parts on FPC has become finer, and as the pitch becomes finer, there arises a problem such as short-circuit between adjacent connection parts. Also, when the pitch is fine, the physical strength of the portion for connection is low and connection stability is disadvantageously poor. Therefore, it has been required to develop a method for mutually connecting the circuit boards, which method is free from the problem of short-circuit and assures high reliability of connection.
Connection through an adhesive has been used for the mutual connection of circuit boards of a fine pitch. Such a method includes the steps of arranging a thermo-softening thermosetting adhesive between two circuit boards, thermally press-bonding the circuit boards to first soften or fluidize the adhesive and to bring the connection parts into mutual contact, further conducting heating to cause setting and establishing the mutual connection of the circuit boards. According to such a method, since the adhesive is sandwiched between the respective connection parts, the problem of short-circuit does not occur even when the pitch between the connection parts is fine. Since the connection parts are supported and fixed by the adhesive film, connection is not released by external stress and connection reliability can be improved. To minimize damage of the circuit boards during thermal press-bonding of the connection parts with one another, thermal press-bonding is preferably carried out at a low temperature and a low pressure. However, a thin layer of the adhesive is formed between the connection parts during thermal press-bonding at a low temperature and a low pressure particularly when the heat softening temperature of the adhesive is low, and connection between the connection parts is not always easy. To solve this problem, Patent Document 1 (Japanese Unexamined Patent Publication (Kokai) No. 2002-97424) proposes to apply concavo-convexity processing to at least one of the surfaces of each connection part of the circuit board to be connected. This technology can improve a contact pressure at the convexity portion during thermal press-bonding, enhances reliability of connection and eventually prevents connection defects.
The means described above requires the application of the processing such as emboss processing at the connection parts of the circuit boards and invites the addition of the production steps. Therefore, development of the method that does not require the additional step and moreover can acquire the effect at least equivalent to that of the means described above has been desired.